fix(rdpdr): fix incorrect padding when parsing NDR strings (#1015)

When parsing Network Data Representation (NDR) messages, we're supposed
to account for padding at the end of strings to remain aligned on a
4-byte boundary. The existing code doesn't seem to cover all cases, and
the resulting misalignment causes misleading errors when processing the
rest of the message.

Author: rhammonds-teleport

crates/ironrdp-rdpdr/src/pdu/esc/ndr.rs

@@ -80,17 +80,21 @@ pub fn ptr_size(with_length: bool) -> usize {
 /// offset fields prefixing the string, as well as any extra padding for a 4-byte aligned
 /// NULL-terminated string.
 pub fn read_string_from_cursor(cursor: &mut ReadCursor<'_>, charset: CharacterSet) -> DecodeResult<String> {
+    const ALIGNMENT: usize = 4;
     ensure_size!(ctx: "ndr::read_string_from_cursor", in: cursor, size: size_of::<u32>() * 3);
-    let length = cursor.read_u32();
+    let _length = cursor.read_u32();
     let _offset = cursor.read_u32();
     let _length2 = cursor.read_u32();
 
     let string = utils::read_string_from_cursor(cursor, charset, true)?;
 
     // Skip padding for 4-byte aligned NULL-terminated string.
-    if length % 2 != 0 {
-        ensure_size!(ctx: "ndr::read_string_from_cursor", in: cursor, size: size_of::<u16>());
-        let _padding = cursor.read_u16();
+    let mut pad = cursor.pos();
+    let size = (pad + ALIGNMENT - 1) & !(ALIGNMENT - 1);
+    pad = size - pad;
+    if pad > 0 {
+        ensure_size!(ctx: "ndr::read_string_from_cursor", in: cursor, size: pad);
+        cursor.advance(pad);
     }
 
     Ok(string)